1. Field of the Invention
The present invention relates to evaluating stress on a surface without contacting the surface.
2. Description of the Related Art
Many advanced defense missile systems use an infrared (IR) seeker for the purpose of identifying and tracking the intended target of interest. Due to the nature of the aerothermal flight environment, the protective IR transparent window must be able to survive extremely high thermal stresses (>100 MPs) in order to prevent catastrophic failure. In many missile systems, the window material of choice has been crystalline sapphire, which has both optical and mechanical properties that are suitable over a wide range of operational flight conditions.
To assure the safe operation of a seeker window, the performance of the IR window under realistic stress and temperature conditions typically is examined. In this examination, the threshold limits of the material making up the window may be determined. Since the conditions encountered in use typically are extreme, the same conditions typically are encountered in testing. Testing often occurs in a wind tunnel. However, the measurement of sapphire window strain in hypersonic wind-tunnel applications is very difficult. Aerothermal heating and shear usually preclude the mounting of common strain gauges on the front side of windows under test. Back-side mounting is complicated by the extreme temperatures commonly seen by these windows.
In many test simulations, sample temperatures can easily exceed 500 degrees C. and can extend to 1000 degrees C. Temperatures of this magnitude prohibit the use of conventional, direct-contact strain gauge transducers. Along these lines, strain gauge adhesives typically break down at temperatures in the vicinity of 320 degrees C.
Mounting strain gauges on the back-side of the windows also does not allow the measurement of front-surface stresses. The physical size of strain gauges reduces spatial resolution and does not allow for a high density of measurements. Strain gauges are intrusive and can affect thermal gradients and, thereby, local strain gradients on the material under test. Crystalline windows are also commonly used with corrosives where strain gauges are attacked by the surrounding media.
Optical fluorescence provides an alternative approach to direct contact probing. Optical fluorescence relies on the ability to generate emission from ions such as chromium, magnesium, and vanadium that are embedded in a crystalline lattice of window materials. For example, it is known that chromium ions in crystalline sapphire produce a narrow-band fluorescence doublet in the red region of the spectrum. The doublet is sensitive to both temperature and stress in the sample. These two intense emission lines are termed the R-fluorescence lines.
The effect of an applied stress to a sapphire window is the distortion of the crystal field surrounding the chromium ion. The distortion changes the potential energy of the ion and, hence, the emission wavelength of the fluorescence radiation. Thus, the effect of stress can be quantitatively calibrated as a shift in the characteristics of the R-fluorescence lines and used as a non-contact probe of stress in sapphire windows.